In recent years, the development of fuel cells that have excellent generation efficiency and emit no CO2 has proceeded with the goal of global environment protection. A fuel cell generates electricity from H2 and O2 through an electrochemical reaction. The fuel cell has a sandwich-like basic structure and includes an electrolyte membrane (ion-exchange membrane), two electrodes (fuel electrode and air electrode), gas diffusion layers for O2 (air) and H2, and two separators.
Fuel cells are classified as phosphoric acid fuel cells, molten carbonate fuel cells, solid oxide fuel cells, alkaline fuel cells, and polymer electrolyte fuel cells (PEFCs; proton-exchange membrane fuel cells) according to the type of electrolyte membrane used. Development of each of these types of fuel cells is ongoing.
Of these fuel cells, polymer electrolyte fuel cells have, for example, the following advantages over other fuel cells.
(a) The fuel cell operating temperature is about 80° C., which enables electricity generation at a remarkably low temperature.
(b) The fuel cell body can be made lighter and smaller.
(c) The fuel cell can be started promptly, and has high fuel efficiency and power density.
Use of polymer electrolyte fuel cells is therefore anticipated in electric vehicle power sources, home or industrial stationary generators, and portable small generators.
A polymer electrolyte fuel cell generates electricity from H2 and O2 via a polymer membrane. As illustrated in FIG. 1, a membrane-electrode joined body 1 is sandwiched between gas diffusion layers 2 and 3 (for example, carbon paper) and separators 4 and 5, forming a single component (referred to as a “single cell”). An electromotive force is generated between the separators 4 and 5.
The membrane-electrode joined body 1 is called a membrane-electrode assembly (MEA). The membrane-electrode joined body 1 is an assembly of a polymer membrane and an electrode material such as a carbon black-supported platinum catalyst on the front and back surfaces of the membrane, and has a thickness of the order of tens to hundreds of micrometers. The gas diffusion layers 2 and 3 are often integrated with the membrane-electrode joined body 1.
In actual use of polymer electrolyte fuel cells, tens to hundreds of single cells such as the above are typically connected in series to form a fuel cell stack and are used in this form.
The separators 4 and 5 are required to function not only as
(a) partition walls separating single cells, but also as
(b) conductors carrying generated electrons,
(c) air passages 6 through which O2 (air) flows and hydrogen passages 7 through which H2 flows, and
(d) exhaust passages through which produced water or gas is discharged (the air passages 6 and hydrogen passages 7 also serve as the exhaust passages).
Therefore, the separators 4 and 5 need to have excellent durability and electrical conductivity.
A durability of about 5,000 hours is expected in the case of a polymer electrolyte fuel cell that is used as a power source in an electric vehicle, whereas a durability of about 40,000 hours is expected in the case of a polymer electrolyte fuel cell that is used as a home stationary generator or the like. Therefore, the separators are required to have sufficient corrosion resistance for withstanding long-term generating, because dissolved metal ions due to corrosion may reduce the proton conductivity of the polymer membrane (electrolyte membrane).
Regarding electrical conductivity, the contact resistance between the separator and the gas diffusion layer is preferably as low as possible, because an increase in contact resistance between the separator and the gas diffusion layer lowers generation efficiency of the polymer electrolyte fuel cell. In other words, lower contact resistance between the separator and the gas diffusion layer contributes to better power generation characteristics.
Polymer electrolyte fuel cells using graphite as separators have already been commercialized. Separators made of graphite are advantageous in that contact resistance is relatively low and also that corrosion does not occur. However, separators made of graphite easily break by impact, and so are disadvantageous in that size reduction is difficult and processing cost for forming air passages and hydrogen passages is high. These drawbacks of separators made of graphite hinder the widespread use of polymer electrolyte fuel cells.
Attempts have been made to use a metal material as the separator material instead of graphite. In particular, various studies have been conducted to commercialize separators made of stainless steel, titanium, a titanium alloy, or the like for enhanced durability.
For example, Patent Literature (PTL) 1 discloses a technique of using, for separators, a metal such as stainless steel or a titanium alloy that easily forms a passive film. The formation of the passive film, however, causes an increase in contact resistance, and leads to lower generation efficiency. These metal materials have thus been pointed out to have problems that require mitigation such as high contact resistance and poor corrosion resistance as compared with graphite materials.
PTL 2 discloses a technique of plating the surface of a metal separator such as an austenitic stainless steel sheet (SUS304) with gold to reduce the contact resistance and ensure high output. However, a thin gold plating is susceptible to formation of pinholes, whereas a thick gold plating is problematic in terms of cost.
To solve these problems, we previously proposed, in PTL 3, “a metal sheet for a separator of a polymer electrolyte fuel cell wherein a film of a Sn alloy layer is formed on the surface of a metal substrate and the film contains conductive particles”.